The increasing sophistication of semiconductor technology has resulted in a significant shift away from aluminum as the dominant metal in multi-level metallization processes. Copper has been proven in limited production volumes, and is likely to become the metal of choice in future designs. The differences between aluminum and copper present unique challenges and opportunities for manufacturers of metrology systems. In parallel with the development of copper-based metallization processes is the ongoing reduction in minimum line widths. Both singularly and together these two factors present a number of challenges and problems in the field of non-destructive optical metrology.
Presently, there are several methods for depositing thin films of copper for back end of the line (BEOL) metallization processes. The dominant method currently involves depositing a seed layer of copper on top of a barrier metal such as tantalum, then electroplating a thick layer of copper on top of the seed layer. Once deposited, it is important to be able to verify that the deposited metal is within specifications. Therefore, what is needed is a technique to accurately measure the thickness of metal films.
The traditional method for measuring the thickness of copper films is the four-point probe method. With this technique, originally developed in the 1950s, an array of four pointed probes arranged in a straight line is pressed into the conductive copper film. Current is applied to one outer probe, and returned via the opposite outer probe. A measurement of the voltage between the middle two probes is combined with the amount of current and knowledge of the bulk resistivity of the film to determine the thickness of the film. However, this method requires that the probes penetrate the surface of the conductive layer. Doing so causes scratches, and can also cause small amounts of particulates to form that can cause defects elsewhere on the wafer. Additionally, this method requires a priori knowledge of the bulk resistivity of the film. The bulk resistivity depends in part on the grain structure and orientation of the grains in the metal, and a problem that has challenged process engineers working on copper metallization processes is that the grain structure and orientation of the grains in copper changes as a function of time, even if the metallized wafers are left at room temperature. Therefore, what is also needed is a non-contact technique to measure metal films.
A measurement using an existing metrology system produces a signal that results from inducing changes in reflectivity caused by stress fields propagating into the sample. As the stress field encounters acoustic impedance mismatches, a portion of the stress field scatters back toward the surface. A probe beam monitoring the time dependent change in reflectivity detects changes in the reflectivity as the stress fields propagate. However, this signal is small, and the presence of noise complicates analysis methods. A particularly problematic type of noise is low-frequency (within the data acquisition bandwidth) “multiplicative noise” that arises from power fluctuations of the laser, vibration of the sample or system optics and temperature fluctuations or air currents in the system that steer the beams. These 1/f noise sources can be reduced by proper laser selection, vibration isolation of the measurement system, and shielding the air currents around the system. However, the resulting level of noise may still be too large to make repeatable measurements on certain metal samples with extremely thin layers or thin buried layers. Thus, what is also needed is an improved technique to accurately extract film thicknesses from measured data when there is noise in the data. A technique to suppress 1/f noise present in the measurement data is needed as well.
Furthermore, existing methods for extracting layer thickness information from the measurement data are adequate for many cases. However, if there is a particularly thin layer adjacent to a thick layer, the high-frequency acoustic signal reflecting from the thin layer may be severely attenuated and easily obscured by noise. What is thus also needed is a technique to resolve fine structure in the measurement data.
Metrology systems have been devised that are intended to improve upon limitations described in the foregoing. In many cases, the acoustic features of interest ride on a large amplitude slowly-varying thermal response or “background signal.” Time-differentiating the total signal numerically can remove the background but does not improve the acoustic signal to noise content. On the other hand, if the time-differentiation is done optically at a frequency above the 1/f noise region (several kHz for the air currents and vibration, up to about 1 MHz for the laser noise), the background signal is “stripped” without contributing to the multiplicative noise. Thus an optical time-differentiator can decrease noise in the data, leading to improved measurement repeatability for a given data averaging time.
Methods for measuring the derivative with respect to time of the response of a sample as a continuous function of time are known. In these methods, path length may be adjusted by means of the mechanical delay line, and in addition path length may be rapidly modulated at a frequency fpath by a small amount. One method described for modulating was to mount a mirror in one of the beam paths on a piezoelectric transducer, the mirror oscillating along an axis parallel to the direction of propagation of the beam. In this method, the frequency fDIFF, and the maximum modulated time delay is limited by the characteristics of the piezoelectric actuator. A typical maximum fDIFF is a few kHz, and a typical modulated delay is 100 fsec. By folding the beam path to effect multiple reflections from the oscillating mirror and mounting the mirror on a cantilever, the modulated delay may be increased to several picoseconds. In practice, however, this may cause the beam to “wander” significantly in response to the modulation, and furthermore may generate significant vibrations in the measurement apparatus, thus degrading the repeatability and accuracy of a measurement. Another scheme employs a rapidly spinning transparent block. Yet another scheme employs a rapidly spinning wheel with transparent sections which provide two or more optical path lengths. All of these schemes suffer from drawbacks similar to those described for the piezoelectric actuator, and in addition may significantly modulate the position, direction, quality, intensity and polarization of the laser beam.
Exemplary United States patents that are relevant to the system(s) of interest herein are now introduced.
U.S. Pat. No. 6,008,906, “Optical method for the characterization of the electrical properties of semiconductors and insulating films,” describes a method for characterizing a sample that includes the steps of (a) providing a semiconductor material; (b) applying at least one of an electric field, a pulsed or cw light source, a change in temperature and/or a change in pump pulse intensity to the semiconductor material; (c) absorbing pump light pulses in a portion of the semiconductor material and measuring changes in optical constants as indicated by probe light pulses applied at some time t following the absorption of the pump light pulses; and (e) associating a measured change in the optical constants with at least one of a surface charge, dopant concentration, trap density, or minority carrier lifetime.
U.S. Pat. No. 4,710,030, “Optical generator and detector of stress pulses,” describes an optical stress pulse generation and detection system for non-destructively measuring physical properties of a sample. This system uses a pump beam having short duration radiation pulses having an intensity and at least one wavelength selected to non-destructively generate a stress pulse in a sample. The system directs the non-destructive pump beam to a surface of the sample to generate the stress pulse. The optical stress pulse generation and detection system also uses a probe radiation beam and guides the probe beam to a location at the sample to intercept the stress pulse. The change in optical constants induced by the stress pulse is detected by observing the probe beam after it intercepts the stress pulse.
U.S. Pat. No. 5,379,109, “Method and apparatus for non-destructively measuring local resistivity of semiconductors,” describes an apparatus for non-destructively measuring the resistivity of a semiconductor, such as InP. The system has light sources for illuminating a pre-selected portion of the semiconductor with first and second light beams, each of a pre-selected single wavelength. The first light beam operates to excite the semiconductor by photo-injecting carriers, and the second light beam bombards the local portion of the semiconductor with a pre-selected photon energy. The system measures a fractional change in reflectance of the second light beam responsive to the first light beam, and records this fractional change in reflectance for various values of photon energy of the second light beam, to generate a photoreflectance line-shape. The photoreflectance line-shape is used to calculate a photoreflectance line-shape phase angle, which is used to determine the resistivity of the pre-selected portion of the semiconductor.
U.S. Pat. No. 5,546,811, “Optical measurements of stress in thin film materials”, describes a method for determining the residual stress in an unsupported region of a thin film. The method includes the steps of (a) optically exciting the film with a spatially and temporally varying optical excitation field to launch counter-propagating acoustic modes along at least one wave vector; (b) diffracting a portion of an optical probe field off the excited acoustic modes to generate a time-dependent signal field at the excitation wave vector; (c) detecting the signal field to generate a time-dependent, light-induced signal; (d) analyzing the light-induced signal to determine the frequencies of the acoustic modes; (e) partially determining the dispersion of at least one mode; and, (f) comparing the measured dispersion to that calculated using a mathematical model to allow the residual stress properties of the unsupported region of the film to be determined.
U.S. Pat. No. 5,693,938 “Optical probe microscope having a fiber optic tip that receives both a dither motion and a scanning motion, for nondestructive metrology of large sample surfaces”, describes an optical probe microscope that includes an optical fiber oriented in a vertical direction. The fiber has a tip that emits light onto a horizontal surface of a sample to be measured. This surface can have both desired and undesired departures from planarity. An electromechanical device for imparting dither motion to the fiber tip is superposed on another electromechanical device for imparting two-dimensional horizontal scanning motion to the fiber tip. The dither motion has a much higher frequency than that of the scanning motion. Between successive scans, another device moves the sample itself from one horizontal position to another. A microscope receives the optical radiation either transmitted or reflected by the sample surface. The microscope forms a (magnified) image of this received optical radiation on the surface of an optical image position detector. The surface of this detector has a relatively large area compared with that of the (magnified) image. The resulting electrical signal developed by the detector provides desired information concerning the scanning position of the fiber tip. Also, this electrical signal is processed and fed back to a vertical pusher that maintains constant the distance of the fiber tip from the sample surface.
U.S. Pat. No. 6,038,026, “Apparatus and method for the determination of grain size in thin films,” describes a method for the determination of grain size in a thin film sample having steps of measuring first and second changes in the optical response of the thin film, comparing the first and second changes to find the attenuation of a propagating disturbance in the film and associating the attenuation of the disturbance to the grain size of the film. The second change in optical response is time delayed from the first change in optical response.
U.S. Pat. No. 5,959,735, “Optical stress generator and detector,” describes a system for the characterization of thin films, as well as interfaces between thin films, through measurements of their mechanical and thermal properties. In the system light is absorbed in a thin film or in a structure made up of several thin films, and the change in optical transmission or reflection is measured and analyzed. The change in reflection or transmission is used to give information about the ultrasonic waves that are produced in the structure. The information that is obtained can include (a) determination of the thickness of thin films with a speed and accuracy that is improved compared to earlier methods; (b) a determination of the thermal, elastic, and optical properties of thin films; (c) a determination of the stress in thin films; and (d) a characterization of the properties of interfaces, including the presence of roughness and defects.
U.S. Pat. No. 5,844,684, “Optical method for determining the mechanical properties of a material,” describes a system and method for characterizing a sample. The method includes steps of (a) acquiring data from the sample using at least one probe beam wavelength to measure, for times less than a few nanoseconds, a change in the reflectivity of the sample induced by a pump beam; (b) analyzing the data to determine at least one material property by comparing a background signal component of the data with data obtained for a similar delay time range from one or more samples prepared under conditions known to give rise to certain physical and chemical material properties; and (c) analyzing a component of the measured time dependent reflectivity caused by ultrasonic waves generated by the pump beam using the at least one determined material property. The first step of analyzing may include a step of interpolating between reference samples to obtain an intermediate set of material properties. The material properties may include sound velocity, density, and optical constants. In one embodiment, only a correlation is made with the background signal, and at least one of the structural phase, grain orientation, and stoichiometry is determined.
A further example of a photoacoustic system is provided in the article “Picosecond Ultrasonic Study Of Mo/Si Multilayer Structures Using An Alternating-Pump Technique.” Nen-Wen Pu et al. in Applied Physics Letters, Volume 74, Number 2, 11 Jan. 1999, pgs 320–322.
The system disclosed in this article makes use of a pump-and-probe transient reflectivity technique in which the acoustic waves are impulsively excited by optical absorption of an ultrashort “pump” laser pulse and detected as a reflectivity change of the time-delayed “probe” laser beam. The article discloses use of an acousto-optic modulator (AOM) and other components to provide for enhanced signal to noise ratios, and improved sensitivity. However, as this system relies upon acousto-optic diffraction of the pump beam, the ability to take measurements at different wavelengths is limited. More specifically, limitations of this system include the need to vary the pump beam diffraction angle with wavelength, and the limited useful wavelength range of commonly available acousto-optic materials (e.g. TeO2, SiO2). Therefore, the versatility of this system is also limited.
As improving semiconductor technology has driven toward ever thinner layers of varied materials, frequently appearing in the presence of thick layers, a challenge is presented to existing metrology systems. What are needed are improved metrology systems for measurement of thin layers of films, such as the one presented herein.